1. Field of the Invention
The present invention relates to methods for manufacturing multi-junction solar cells.
2. Description of the Related Art
Single-junction solar cells each comprising a single pn junction have been known. However, the generation efficiency of the single-junction solar cell has a theoretical limit determined by the bandgap Eg of a semiconductor material used as a raw material. It is thus known that regardless of whatever semiconductor material is used, the generation efficiency achieved under ground-based solar irradiation conditions is limited to about 30%.
Thus, to offer generation efficiency higher than that of the single-junction solar cell, proposal has been made of a multi-junction solar cell formed by stacking a plurality of solar cells each having a pn junction. The most simple known multi-junction solar cell is a 2-junction solar cell 11 shown in FIG. 3 (see, for example, Japanese Patent Laid-Open No. 9-64386).
The 2-junction solar cell 11 has a bottom cell 13 and a top cell 14 stacked on a back electrode 12 in this order by means of epitaxial growth. A surface electrode 15 is stacked on the top cell 14. In the 2-junction solar cell 11, the bottom cell 13 comprises a GaAs pn junction, whereas the top cell 14 comprises an InGaP pn junction.
The multi-junction solar cell is known to be based on the principle that the available area of a solar spectrum is increased by joining semiconductors with different bandgaps and to be more efficient with more junctions. For example, 4-junction solar cells have been simulated and high efficiency solar cells are expected.
A combination of semiconductors with different bandgaps normally has different lattice constants. Thus, joining semiconductors with significantly different lattice constants may disadvantageously result in a defect in the junction interface between the semiconductors, preventing an increase in efficiency. This problem is due to the fact that a large number of dangling bonds present in the defect urge excitations excited by solar light to be recombined, thereby power cannot be taken out.
As a result, the combination of semiconductors constituting the multi-junction solar cell is limited to the combination of materials having different bandgaps but similar lattice constants (what is called a well lattice matched combination). When taking further into account the matching with the solar spectrum, possible combinations of semiconductors include a combination of InGaP, GaAs, and Ge, a combination of InGaP, InGaAs, and Ge, and the like. Consequently, the multi-junction solar cell is limited to a 3-junction type.
Among these semiconductors, GaInNAs has been gathering much attention as a material having a bandgap of about 1 eV while maintaining lattice matching with Ge and GaAs. Continuous studies have thus been conducted on a 4-junction solar cell comprising a combination of InGaP, InGaAs, GaInNAs, and Ge (see, for example, J. F. Geisz, D. J. Friedman, C. Kramer, A. Kibbler, and S. R. Kurtz, “New Materials for Future Generations of III-V Solar Cells”, NREL/CP-520-25631, National Renewable Energy Laboratory, December 1998).
However, GaInNAs may cause phase separation or other defects. Thus, with GaInNAs, there is a disadvantage that it is difficult to grow crystals in implementing a 4-junction solar cell.
Further, the performance of the 3-junction solar cell has been evaluated with a device with an area of at most 4 cm2. It is known that the efficiency of the 3-junction solar cell decreases with increasing area of the device (see Akira Ohmae, Yukiko Shimizu, and Yoshitake Okada, “GaInNAs for Multi-Junction Tandem Solar Cells”, Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World conference on Volume 2, 12-16 May 2003). The reduced efficiency is expected to result from a possible defect in the vicinity of the junction interface caused by the different lattice constants of the semiconductors regardless of whether the solar cell is of the 2- or 3-junction type.
Therefore, the multi-junction solar cell has the disadvantage of making an increase in the area of the device difficult.